Flame spray pyrolysis (WO2005/103900, EP1378489) represents a cost efficient process for the manufacturing of high quality metal oxide nano-powders. The resulting powders show a very high surface area together with high material purity. The process further gives the possibility of producing mixed metal oxides with a high homogeneity both in chemical composition and powder particle size. The powders are mostly un-agglomerated and of very narrow size distribution. It was recently shown (Loher et al. 2005, Huber et al. 2005, Grass and Stark 2005) that the process is also capable of producing salts such as calcium-phosphates and fluorides.
Metal, non-oxidic ceramic and reduced metal oxide nano-powders which are currently not available from flame spray pyrolysis are of large industrial interest: Besides giving very colourful materials for use in pigments (Bahador 1995), reduced metal oxides exhibit semi-conducting properties (Lou et al. 2005) and a high ion conductivity of interest for electronic applications and solid state fuel cells. Non-oxidic ceramics such as tungsten-carbide, cobalt-nitride and many others display excellent mechanical properties (GB696589) such as very high hardness and temperature resistance making them of interest for high duty applications such as cutting tools and protective coatings. Nano-sized metal powders such as iron, steel, copper, cobalt and others are of interest for powder metallurgy. Further, these materials exhibit size dependent characteristics (Modrow et al. 2005) such as enhanced electronic, magnetic (Kodama 1999) or mechanical properties giving them manifold applications in the electronic and machining industry. All three groups of materials have applications as reactive surfaces, as ceramics, building materials and in heterogeneous catalysis, especially when the particles are of small size exhibiting large surface areas. Two selected examples of catalysts of interest are tungsten-carbide for platinum-like catalysis such as hydrogenation (Levy and Boudart 1973) and metal nitrides as well as alloys of metal nitrides for hydrodenitrogenation (Milad et al. 1998; Wang et al. 2005). Further applications include low melting alloys for interconnects in electronics (Li et al. 2005).
Currently metal powders and alloys are produced by a series of different processes depending on the necessary product size and purity. For large metal particles (above 1 micrometer) atomization of liquid metal using a nozzle, disk or cup (see e.g. US 2005/009789 and references therein) is used as an efficient low cost method. Particle size is strongly limited by the smallest liquid droplet which can be formed. Smaller particles can be formed by alloy leaching (see e.g. WO 2004/000491). This process is limited to only a few metals and their alloys, results in large amounts of liquid waste and leads to strongly agglomerated particles which have to be de-agglomerated (e.g. by milling). Spherical, un-agglomerated and monodisperse metal nano-particles can also be obtained by wet-phase chemistry (such as Nicolais 2005). Besides the large liquid waste produced by these processes, the application of the produced particles is limited to the liquid phase as it is difficult to dry the powders completely without leaving surfactants and solvents contaminating the residual product. High temperature electronic processes, such as lasers (Dez et al. 2002) and plasma reactors (e.g. U.S. Pat. No. 5,486,675, GB 2 365 876, DE 39 37 740) are used for the fabrication of nanosized metal powders. Due to the high necessary temperatures (several 1000 K), the high cost of electrical energy and low efficiency, these processes remain relatively expensive and complex. A further method for the synthesis for metal nano particles is vapor flow condensation (see Wegner et al. 2002 and references therein). This process however is limited to metals with low vaporisation temperature. Several pyrolysis processes in hot tubes have been reported in the academic literature (Eroglu et al. 1996, Knipping et al. 2004) but all are limited to low production rates. The major disadvantage of these processes is the diffusion and thermophoresis of particles to the tube wall and therefore lowering the process yield making up-scaling difficult. All state of the art processes give particles with a broad size distribution that is undesirable in most applications.
It is therefore of great industrial interest to have a production method which best combines cost efficiency and versatility.